Alarm system



Ap 5, 1938. E. D. cooK ALARM SYSTEM Filed Jan. 31; 19:54

l Patented Apr. `5,1931;

PATENT oFFlcE amsn am srs'rmu Ellsworth D.- Oook. Merchantvillo, N. J., assignor l to Radio Corporation of Amel-Ica, a corpora- 'umu Delaware Application January 31, 1934, ScrlalNo. '709,071

4Claims.

My invention relates to alarmA systems and particularly to systems of the type designed to give an audible or visible indication of the presence of an intruder in a particular protected area.

More specifically, my invention contemplates an alarm system wherein one or more oscillators are utilized, and the presence of an intruder into the protected area disturbs a predetermined bal- 'ance between the oscillators, with the result that the frequency of a beat-note is changed.

Systems of the aforesaid type, heretofore known to me, have had one rather serious drawback, namely, the tendency of either or both oscillators to drift away from their assigned frequencies. Attempts have been made to overcome this diillculty, usually by providing means, responsive to weather conditions and the like, which would adjust one or both oscillators back to its predetermined frequency at a rate commensurate., with the rate of occurrence of the natural phenomena. That is to say, frequently correction systems depended for their action upon very exact and nice adjustments whereby the frequency of either or both oscillators could be maintained stable.

Obviously, it is somewhat difficult to maintain a plurality of oscillators functioning with but small changes in their respective frequencies and it is, accordingly, an object of my invention to provide vimproved frequency drift compensating means.

In addition to the foregoing, -alarm systems constructed according to the teachings of' the prior` art, in general, utilize 'correction devices which are responsive to frequency drift but the correction introduced thereby is not strictly proportional to the departure from normal of the corrected devices. It is, therefore, a further object of my invention to provide frequency-drift 40 compensating means that shall apply a correction strictly in accordance with the amount, duration and sense of a'departure from normal of the system.

'Ihe foregoing objects and other objects ancillary thereto I accomplish, not by working directly upon the output of one of the oscillators but by providing means forkeeping constant the dierence frequency betwaen two oscillators in spite of the drifting of either or both of them by reason of natural causes. Specifically, I provide two oscillators having a predetermined normal frequency-difference and utilize a departure froni normal of the frequency difference for the purpose of applying a correction to one of them.

More particularly, I provide means for detecting the beatfrequency and impress the detected frequency, preferably 1000 cycles, after amplification, upon two filters, one a sharply tuned highpass lter and the other a sharply tuned low-pass filter. The constants of the lters are so chosen that the characteristic curves intersect at an acute angle at a frequency corresponding to the beat frequency chosen, namely 1000 cycles.

r'.ll'he output from each filter is rectified andthe rectified currents are applied to a balanced relay device which, in tum, through the use of a piurality of slow-closing, quick-opening relays, controls a reversible capacitor motor. The motor is coupled through a reduction gear to a Vernier tuning condenser associated with one of the oscillators and, by properly choosing the polarity of the connections between the several devices mentioned, the motor may be caused to automatically adjust' the tuning condenser to compensate for any drift of the beatfrequency above or below the normal beat-frequency.

Further, in accordance with my invention I utilize the beat frequency, after passing through a sharply tuned band-pass filter, for the purpose of keeping open the contacts of a relay device which, when closed, complete a circuit for energizing a warning signal.

'Ihe novel features of my invention are set forth with particularlty in the appended claims. The invention itself, however, both as to its 4organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specic embodiment when read in connection with the accompanying drawing in which Fig. 1 is a complete diagrammatic view of an alarm system including a preferred embodiment of my invention, and

Fig. 2 shows a series of curves illustrating certain filter characteristics to which reference hereinafter will be made.

Referring now to the drawing, an alarm system lconstructed according to my invention includes a plurality of oscillators I and 3. The frequency-determining capacity of the first mentioned oscillator is constituted by the stray ca.- pacity between a ground lead, which may be an extended wire, 5, and an antenna 1, which also may be a wire extended along a wall, for example,'parallel to the ground connection. The frequency of the second mentioned oscillator 3, is determined by a variable tuning condenser 9, and a variable vernier condenser Il, to which reference will later be made in detail,

nmcillsimmnnuplcamnlinxcimnii including a plurality or coupling coils is nnn is,

one terminal of the circuit being conductively oonnectedtothegridofadetectortubc Il.and theotherterminalleadingtoacommoncathode connection Il. In order to prevent radiation of energy from theoacillator I, I ilnd it highly desirable, in certain cases. to reduce the frequency thereof to the neighborhood of -30,000 cycles per second and interpose Va frequency multiplier Il between its output circuit and the link circuit coupled tc the input circuit of the detector I1. This modiilcation is alternative and need not be illustrated in more detail since frequency multipliers,perse,areoldandwellknowntothose skilledintheart. Itistobeunderstood,how ever, that following description, and the claims, are intended to cover the system either with or without the frequency multiplier since its presence in no way alters the flmdamental action thereof In normal operation of the system the frequency of the two oscillators is so chosen that the beat produced thereby is i000 cycles although it is, of course. to be understood that I am not limited to any particular beat frequency nor am I limited to any particular pair of frequencies for the several oscillators.

After rectification in the detector tube Il, the beat frequency is impressed upon an ampliiler tube II, through a tuned band-paas illter 2l, the characteristic curve of which is shown at A in Fig. 2 of the drawing. Subsequent to amplification the beat frequency is utilized to determine the action of an alam-circuit controlling device.

Although not limited thereto, I nnd it desirable to utilize for the controlling device a thermionic tube 2l, such as that commercially known as a 'Ihyratron," hereinafter designated the relay tube. The tube in question includes. within an evacuated envelope, a control grid 21, a thermionically active cathode 2l, an anode ll, and a heater Il, to which energizing potential may be applied from any commercial alternating current source such as through the utilization of a transformer 3l.

Alternating annue potential is applicato the 4tube from the secondary of the transformer over a circuit including the winding of a relay 81. The armature It, of the relay is electrically connected to the same terminal of the transformer-secondary which is connected to the anode of the relay tube. When the relay winding is de-energized to release the armature, a circuit is completed from the said terminal through the armature. a contact member II. the energizing winding of a bell II, or the like, the contacts and movable member of a normally closed pushbutton switch 41, a resistor l', hold-in winding 5I. and a condenser I4 to the other terminal of the secondary winding.

In the absence of a Proper signal potential upon the grid of the relay tube, such as is applied by the positive half-cycles of the beat frequency, space current does not flow in the device. In this condition the armature ll of the relay drops out, contacting 4 I and the circuit previously described in detail is completed, which causes an alarm to be given. Condenser I4 is preferably of large capacitance to permit passage of current from the volt A. C.' source when the alarm is thrown in circuit by amature 39. A battery I2 is provided in shunt with condenser I4 in order that the alarm It may nevertheless be energized in the event of failure of the 110 volt source.

wnennenerninatinpnmd'tnecnrieniih" .closed push-button switch I1.

Assuming that both oscillators are so functioning that the beat note of 1000 cycles is being impressed upon the grid of the control tube, the said tube draws suillcient current to maintain the contacts of the relay operi.K If, now, an intruder comes between the two wires, 'I and l. connected to the first oscillator I or comes close to the high potential wire 1, the frequency of the said oscillator is lowered with the 4result that the beat note is reduced in frequency by a deilnite number of cycles.

Referring now to Fig. 2 of the drawing, the curve A exempliiles the characteristics of the band-pass iliter 2l. Prom this curve it will be noted that the transmission of the filter does not vary until the beat-frequency departs from 1000 cycles by some amount in excess of one half the range covered by the ilat top of the curve and that, thereafter, the attenuation is very rapid. At certain points. D and Dl. on the cum the attenuation is so great that the grid of` the relay tube 25 is substantially deprived of excitation with the result that space current stops flowing therein and the relay drops out to give an alarm.

In an actual operative system constructed according to my invention, the departure from normal of the beat frequency necessary to cause an alarm, indicated by the frequency-increment Ai] in the drawing. is of the order of 50 cycles. corresponding to a change in the distributed capacity to ground of the antenna 'I of the order of $6 micromicrofarad. Since the approach of an intruder within several feet of the wire, which was disposed at a heighth of approximately six feet above ground or spaced the same distance from the wire I, caused such change in the distributed capacity, the sensitivity of my improved system is ample.

In the system referred to. the oscillator I funcy tions normally at a frequency in the order of' 100,000 cycles per second and the oscillator I at a frequency in the order of 99.000 cycles per second, but it is to be distinctly borne in mind that the percentage of change in frequency of the oscillator I occasioned by an intruder, is -independent of the frequency thereof.

Inorder that frequency drift, occasioned by changes in potential supply, atmospheric conditions. etc., shall be compensated, I provide further means, responsive to departure from normal of the beat frequency, or adjusting the frequency of the oscillator I. make certain that the frequency-compensa means shall not immediately counteract the effect of the change in beat frequency caused by the momentary presence of an intruder, I introduce time delay means whereby compensation is deferred until a deilnite time interval elapses after the frequencychange has taken place. I also pro/vide means whereby very slow changes in beat-frequency are compensated before they reach such magnitude' as to cause the alarm to be given. In other words,

" referring to Fig. 2, a departure from normal of the beat frequency by an increment, Anf less thanV the departure An* necessary to give analarm, is compensated automatically to keep the variation.l

Specifically, referring again to Fig. 1 of the drawing, for frequency-drift compensation, I couple the input of a further audio-frequency 'ampliiier 63 to the output circuit of the detector I1 and impress the amplified audio frequency through a high-pass filter 55 and a low-pass filter 51 upon two rectifying devices, 59 and 6|, re-

. spectively. The output circuit of the high-pass filter rectifier includes a resistor 62 and one winding 63, of a balanced relay 64, the other winding 65 of the relay and a resistor 66 being included in the output circuit of the low-pass filter rectifier. A front contact 61 and a back contact 69, are associated with the armature 1I of the relay which armature, when the currents in the several windings are substantially balanced or when there is current in neither, remains in the central, open position as indicated in the drawing.

The filters are so designed as to have characteristic curves of the form of curves B and C, shown in Fig. 2, which curves intersect at 1000 cycles, and the slopes thereof at the intersection are quite steep.

The armature of the balanced relay is' connected through a potential source, such as a battery 13, to the junction point 1I, between the windings of a plurality of slow-closing, quickopening relays, 15 and 11. Preferably, these relays when energized require at least one-half minute for closing but open substantially instantly when the windings are de-energized. The several contacts, 61 and 69, associated with the armature of the balanced relay, are connected, respectively, to the outer terminals of the slowclosing relay windings. It will be noted from an inspection of the drawing that the contact of each slow-closing relay is connected to one side of a 110 volt alternating current supply source. 'I'he other side of the source is connected to the junction point between the two windings in a reversible capacitor motor 8l, one winding of the motor having a connection to the other armature, and the two windings being interconnected through a large condenser, 16. T he motor drives the Vernier condenser Il through a reduction gear 62.

Ii' the beat frequency departs from normal by a very slight amount, indicated by the point F or the point E on the curves B in Fig. 2, no correction is necessary and, in fact, such departure is insuillcient, usually to cause operation of the correction means. If, however, it departs therefrom by an abnormal amount, i. e.

' an amount greater than Azf and less than the change caused by an intruder in the frequency of the oscillator 3, correction should take place and this correction-should be continuously -applied just so long as the tendency to drift persists.

To this end, the unidirectional current output from the high-pass lter rectifier 59 is normally balanced against the output from the low-pass lter 6I in the windings of the relay 64. 'I'he resistors 62 and 66, may be utilized for balancing purposes, if desired, or any other well known means may be utilized for ensuring that the armature 1| remains in the mid-position when the output of the detector is at a frequency of 1000 cycles. The armature moves. therefore, to energize one of the slow-closing relays, only when the outputA current from the high-pass filter differs from that of the low-pass filter.

By balancing the output currents from two filters having characteristics such -as shown by the curves B in Fig. 2, extreme sensitivity is obtained and compensation begins within avery short time after the beat frequency has departed from normal an amount, Arf, less than the departure, Alf, necessary to give an alarm. Furthermore, such compensation is continuous just as long as the tendency to drift persists, thus maintaining the system always in condition such that a sudden departure from normal of the beat frequency by an amount equal to or in excess of Alf will cause the giving of analarm. Therefore, depending upon which filter passes the most current, the balanced armature will move either up or down to supply energizing potential to the lower or the upper slow-closing relay winding. I have not attempted to illustrate which winding causes the motor 6I to increase the capacity of the vernier condenser Il, nor which causes it to rotate in the opposite direction to decrease the capacity thereof, the manner in which this may be done being, of course, obvious to anyone skilled in the art. As a result, however. of the connections exemplified by the drawing, any change in the beat frequency, whether the result of natural causes or occasioned by the presence of an intruder, causes the energizatlon of one or the other of the slowclosing relays but since they close slowly, the momentary presence of an intruder occasions no correction of the oscillator although such presence does cause an alarm to be given.

It might be inferred from the foregoing that my invention is.1imited by the specific relay tube 25, and relaycircuits chosen for illustration. This is not the case, however, since many other equivalent devices may be utilized for the purpose of giving an alarm when the audio frequency amplifier 2l is deprived of input excitation through change in the beat frequency. Again, it might be assumed that I am limited to quite high frequencies for the oscillators I and 3. Such is not the case, however, since the operation of my system is substantially independent of the frequencies chosen.

It will be apparent from the foregoing that I have provided -an improved alarm system which has a number of obvious advantages, and among these advantages especial attention is called to the fact that I have obviated the necessity of providing complicated circuits for keeping either or both oscillators functioning at definite frequencies. In fact, either oscillator may drift away from its assigned frequency in either direction but the action of the lcorrective system, since the said system is controlled by the beat note, will always bring the corrected oscillator to such a frequency as to give the correct beat note.

Although I have chosen certain specific pre-l oscillation generators operative to provide a beat signal of predetermined frequency, said oscillators being susceptible to normally slow variationstendingtoaltsrthefrequencyofsaidbeat signal, means by said beat signal for adjusting the freqncy of one of said oscillators to compensate for said' normal variations in the frequency of said beat signal, means for delaying the response of said last mentioned means totherebydistinluiahbetweenslowandrapid frequency variations. capacity elements connectedtotheotherofsaid oscillators forproducing abnormally rapid changes in the frequency of said beat signal in the presence of a foreign body when brought into predetermined relation c with said capacity elements, an indicating device, and means responsive only to said abnormal changes in the frequency of said beat signal for actuating said indicating device.

3. In a device of the character described. means for generating a signal current of predetermined frequency. said generating means being subject to normally slow frequency variations which tend to alter said predetermined frequency. an alarm, means responsive to currents of a range of frequencies including said predetermined frequency for maintaining said alarm in responsive condition, means responsive to the presence of foreign bodies for changing the frequency of said currents of predetermined frequency to thereby actuate said alarm responsive means. means for compensating for said normally slow frequency variations and actuated by said variations to v thereby maintain said ed frequency. and means which is slow to operate but quick to release for delaying the action of said compensating means so tint the said frequency compensating means will only respond to said normally slow variations whereby said compensation meansisslowlyoperatedtomaintainsaidsignal current substantially at said predetermined frequency and quickly released after said compensation has been eected.

3.In` a device oi the character described. al

`pair0foscillatorsforgeneratingabeatfrequency current. said oscillators being subject to normally slow frequency variations. an alarm, means responsive to said beat frequency current for maintaining said alarm in responsive condition, means responsive to the presence of foreign bodies connectedto one of said oscillators for changing said beat frequency and thereby actuating said alarm. means for compensating for saidnormally slow frequency variations' and actuated by said variations to thereby .maintain said beat frequency, and means which is slow to operate but quick to release for delaying the compensating means will only respond to said normally slow variations whereby said compensation means is slowly operated to maintain said beat frequency current and quickly released after said compensationhas been eifected.

4. Inanalarmsystem.apairofoscillatorsfor generating a beat frequency current.- said beat frequency being subject to normally slow frequency variations, means for compensating for said normally slow variations and actuated by said variations, means which is slow to operate but quick to release for delayins the operation o f said compensating means so that said means are unresponsive to abnormally rapid frequency changes whereby said compensation means is slowly operated to maintain said beat frequency current free from said slow variations and -whereby said compensation means is quickly released after said compensation has been effected. an alarm, means responsive to said beat frequency currents for maintaining said alarm in responsive condition, and means responsive to the presence of a foreign body for causing abnormally rapid variations in said beat frequency to thereby actuate said alarm.

ELIBWORTH D. COOK.

action of said compensating means so that said 

